Exhaust air purifier for internal combustion engine

ABSTRACT

There is provided an exhaust air purifier for an internal combustion engine that reduces the frequency of estimation of an amount of accumulated particulate matter, and hence energy consumption involved in the estimation of the amount. Based on a difference ΔP 1  between the exhaust pressures before and after a DPF  12  and based on a second amount of accumulated PM, QPM  2,  estimated based on a temperature increase arising when the PM is temporarily heated, first map data for estimating a first amount of accumulated PM, QPM1, based on the difference ΔP 1  between the pressures before and after the DPF 12  is corrected so that a amount of accumulated PM when the difference ΔP 1  between the exhaust pressures is obtained equals the second amount of accumulated PM, QPM  2.  This further improves the accuracy of the first amount of accumulated PM, QPM 1,  estimated by “the process of estimating the first amount of accumulated PM” performed subsequently. This makes it possible to more accurately determine whether to perform “the process of estimating the second amount of accumulated PM.”

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2007-096085 filed on Apr. 2, 2007 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to exhaust air purifiers for internal combustion engines and, more particularly, relates to exhaust air purifiers that collect particulate matter from exhaust gas from a diesel engine and remove the particulate matter by burn.

2. Description of the Related Art

Conventionally, this type of an exhaust purifier for an internal combustion engine estimates the amount of accumulated particulate matter collected by a particulate matter collecting unit, and performs a burning process when the estimated amount reaches a predetermined value.

Examples of such a method include one in which the amount of accumulated particulate matter is estimated based on the difference between exhaust pressures upstream and downstream of the particulate matter collecting unit (e.g., refer to Japanese Patent Application Laid-Open No. 2004-340023), and one in which it is estimated based on a temperature increase resulting from the temporary heating of accumulated particulate matter (e.g., refer to Japanese Patent Application Laid-Open No. 2005-226547).

However, the conventional exhaust air purifiers for internal combustion engines suffer from the problems described below. In the former, accumulated particulate matter may or may not be burned, depending on the temperature or flow rate of exhaust gas subject to the burning process. After the burning process is repeated several times, the pressure difference may vary even when the amount of accumulated particulate matter is the same. This makes it difficult to accurately estimate the amount of accumulated particulate matter based on pressure difference alone.

In the latter, albeit the amount of accumulated particulate matter can be estimated with reasonable accuracy, an exhaust temperature increase is controlled by post injection (i.e., injection after the main injection for each combustion), performed in addition to regular fuel injection. This consumes energy each time an amount of accumulated particulate matter is estimated. Accordingly, frequent estimation of the amount of accumulated particulate matter increases energy consumption.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems of the known technology discussed above. It is therefore an object of the present invention to provide an exhaust air purifier for an internal combustion engine, the purifier being designed so as to reduce the frequency of estimation of the amount of accumulated particulate matter, thereby reducing energy consumption involved in the estimation.

In order to achieve the foregoing object, according to a first aspect of the present invention, there is provided an exhaust air purifier for an internal combustion engine, the purifier including: a particulate matter collecting unit that collects particulate matter from exhaust gas from the internal combustion engine; an exhaust pressure detecting unit that detects exhaust pressures before and after the particulate matter collecting unit; an exhaust temperature detecting unit that detects exhaust temperature downstream of the particulate matter collecting unit; a heating unit that increases the temperature of the exhaust gas passing through the particulate matter collecting unit; and a control unit that estimates the amount of accumulated particulate matter collected by the particulate collecting unit, and controls the temperature of the heating unit, the purifier being characterized in that:

the control unit estimates a first amount of accumulated particulate matter based on the difference between the exhaust pressures before and after the particulate matter collecting unit, which difference has been calculated using values detected by the exhaust pressure detecting unit, and based on first map data pre-stored in a memory, which map data represent the interrelation between the amount of accumulated particulate matter in the particulate matter collecting unit and the difference between the exhaust pressures before and after the particulate matter collecting unit;

when the first amount of accumulated particulate matter has reached a threshold, the control unit causes the heating unit to temporarily heating the particulate matter accumulated in the particulate matter collecting unit and also estimates a second amount of particulate matter based on an increase in temperature at the time of heating of the particulate matter, and based on second map data pre-stored in the memory, which map data represent the interrelation between the increases in the temperature at the time of heating of the particulate matter and the amount of accumulated particulate matter in the particulate matter collecting unit; and

based on the second amount of accumulated particulate matter and based on the difference between the exhaust pressures before and after the particulate matter collecting unit, the difference having been calculated based on the values detected by the exhaust pressure detecting unit, the control unit corrects the first map data representing the interrelation between the amount of accumulated particulate matter in the particulate matter collecting unit and the difference between the exhaust pressures before and after the particulate matter collecting unit.

According to the first aspect of the present invention, when the second amount of accumulated particulate matter is estimated based on an increase in temperature at the time of temporary heating of the particulate matter, the first map data for estimating the amount of accumulated particulate matter based on the difference between the pressures of exhaust before and after the particulate matter collecting unit is corrected according to the difference between the exhaust pressures upstream and downstream of the particulate matter collecting unit, and according to the estimated second amount of accumulated particulate matter. In other words, using the first map data and the difference between the exhaust pressures upstream and downstream of the particulate matter collecting unit ensures more accurate estimation of the first amount of accumulated particulate matter in the subsequent estimation. Accordingly, a comparison between the first amount of accumulated particulate matter, estimated using the first map data, and a predetermined threshold makes it possible to correctly determine whether to estimate the second amount of accumulated particulate matter based on the increase in the temperature at the time of temporary heating of the accumulated particulate matter. This reduces the frequency of estimation of the second amount of accumulated particulate matter, and hence energy consumption involved in the estimation of the second amount.

In the exhaust air purifier according to the invention, when a second amount of accumulated particulate matter to be temporarily heating is estimated, first map data for estimating a first amount of accumulated particulate matter based on the difference between the pressures of exhaust before and after the particulate matter collecting unit is corrected to remove any error in estimation. A comparison between the first amount of accumulated particulate matter, estimated using the corrected first map data, and a threshold, makes it possible to correctly determine whether to estimate the second amount of accumulated particulate matter based on an increase in the temperature at the time of temporary heating of the accumulation of the particulate matter. This reduces the frequency of estimation of a second amount of accumulated particulate matter, and hence energy consumption involved in this estimation of the second amount.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the configuration of an exhaust air purifier for an internal combustion engine in which the present invention is applied;

FIG. 2 is a flowchart illustrating a method for estimating the amount of accumulated PM, which is carried out in an ECU;

FIG. 3 shows first map data representing the interrelation between a pressure difference ΔP and the amount of accumulated PM;

FIG. 4 is a diagram representing the interrelation between an increase in the temperature of the DPF and the amount of accumulated PM; and

FIG. 5 shows second map data representing the interrelation between an increase in the temperature of the DPF and the amount of accumulated PM.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the accompanying drawings, there will be described a preferred embodiment of an exhaust air purifier for an internal combustion engine according to the present invention. FIG. 1 shows the configuration of an exhaust air purifier for an internal combustion engine in which the present invention is applied.

Referring to FIG. 1, there will first be described the configuration of the exhaust air purifier for an internal combustion engine.

An exhaust air purifier 10 for an internal combustion engine includes: a diesel engine 11 for use as an internal combustion engine; a diesel particulate filter 12; a first exhaust pressure sensor 13 and a second exhaust pressure sensor 14; an exhaust sensor 15; a heater; and an ECU 16 serving as a controller.

Specifically, the diesel engine 11 is an engine in which, under the control of the ECU 16, fuel is injected into compressed air in the engine combustion chamber, and then ignited and combusted, thereby producing power. A microprocessor (MPU) in the ECU 16 calculates an appropriate amount of fuel to be injected, by comparing the degree of operation of the accelerator pedal, the number of engine revolutions, and many additional correction coefficients, with characteristic map data stored in a memory. The gas combusted in the combustion chamber is discharged from the engine as exhaust gas.

The diesel particulate filter (hereinafter referred to as “DPF”) 12 collects particulate matter (hereinafter referred to as “PM”) in the exhaust gas in the diesel engine 11 by use of a filter made of, for example, ceramic fiber (silicon carbide monolith); then, when the accumulated PM reaches a predetermined amount, increases the exhaust temperature by use of a heating unit under the control of the ECU 16, and burns and removes the PM through the reaction of a catalyst held by the filter; and recycles the filter. The amount of PM accumulated in the filter is estimated by the ECU 16, which will be described in detail below.

The first exhaust pressure sensor 13 detects exhaust pressure before the exhaust passes through the DPF 12 (i.e., exhaust pressure upstream of the DPF 12). Similarly, the sensor 13 detects exhaust pressure after the exhaust passes through the DPF 12 (i.e., exhaust pressure downstream of the DPF 12). The values detected by the first and second exhaust pressure sensors 13 and 14 are input to the ECU 16 and used to estimate the amount of accumulated PM. Such detection is required because when the amount of PM collected by the DPF 12 increases, the difference in the exhaust gas pressure before and after passing through the DPF 12 increases.

In lieu of the first and second exhaust pressure sensors 13 and 14, an exhaust pressure difference sensor, which detects any difference in the pressure of exhaust before and after passing through the DPF 12, may be used.

The exhaust temperature sensor 15 is disposed downstream of the DPF 12 and detects the temperature TAFT of exhaust that has passed through the DPF 12. The value detected by the exhaust temperature sensor 15 is input to the ECU 16 and used to estimate the amount of accumulated PM, to control temperature for recovering the DPF 12 (in order to prevent an excessive increase in the temperature of the DPF 12, which results in deterioration of the catalyst), and other suchlike purposes.

Under the control of the ECU 16, the heating unit increases the temperature of exhaust gas flowing in the DPF 12. Specifically, fuel injection control is exerted such that in addition to a regular injection of fuel, post injection (i.e., injection after the main injection for each combustion has occurred) takes place. The heating unit is not limited to one designed specifically for fuel injection control but may be any electric heater disposed in close contact with the filter of the DPF 12.

The ECU 16 is formed from a circuit that includes a microprocessor (MPU) operated by a computer program used to control fuel injection, to estimate the amount of accumulated PM, and to burn PM. Specifically, the ECU 16 compares the degree of operation of the accelerator pedal, the number of engine revolutions, and many other additional correction coefficients, with characteristic map data stored in a memory (not shown), then calculates an appropriate amount for injection, and carries out the injection.

In addition, based on a plurality of map data (i.e., numerical value table) stored in the memory, values detected by the first and second exhaust sensors 13 and 14, and values detected by the exhaust temperature sensor 15, the ECU 16 estimates the amount of accumulated PM.

In addition, the ECU 16 exerts temperature control such that when the amount of accumulated PM (hereinafter referred to as a second amount of accumulated PM, QPM2) reaches a predetermined value (threshold), the temperature of exhaust gas passing through the DPF 12 is increased using the heating unit, PM accumulated in the DPF 12 is thereby burned, and the DPF 12 is consequently recycled.

Referring to FIGS. 2 to 5, there will next be described a method for estimating the amount of accumulated PM, which is carried out in the ECU 16.

FIG. 2 is a flowchart illustrating a method for estimating the amount of accumulated PM. FIG. 3 shows first map data representing the interrelation between a pressure difference ΔP and the amount of accumulated PM. FIG. 4 is a diagram representing the interrelation between an increase in the temperature of the DPF and the amount of accumulated PM. FIG. 5 shows second map data representing the interrelation between an increase in the temperature of the DPE and the amount of accumulated PM.

First, the ECU 16 starts instep 1 reading exhaust pressure Pl upstream of the DPF 12, detected by the first exhaust pressure sensor 13, and exhaust pressure P2 downstream of the DPF 12, detected by the second exhaust sensor 14.

In step 2, based on the exhaust pressures P1 and P2 thus read, the ECU 16 calculates the difference ΔP1 between the exhaust pressure upstream of the DPF 12 and that downstream of the DPF 12 from ΔP=|P2−P1|.

In step 3, the ECU 16 initiates “the process of estimating a first amount of accumulated PM,” in which based on the pressure difference ΔP1 calculated in step 2 and the first map data stored in the memory, a first amount of accumulated PM, QPM1, is estimated. “The process of estimating a first amount of accumulated PM” is performed at predetermined intervals.

The first map data, as shown in FIG. 3, represent on a line L1 (OLD) the interrelation between the amount of accumulated PM in the DPF 12 and the difference ΔP between the exhaust pressures upstream and downstream of the DPF 12. The first amount of accumulated PM, QPM 1, in the DPF 12 is estimated by calculating the difference ΔP1 between the exhaust pressures upstream and downstream of the DPF 12.

The amount of accumulated PM, QPM1, represents an amount (g) of accumulated PM per geometric surface 1 m̂2 (wherein ̂ is an operator) where it is assumed that the surface of the cell partition of the filter is smooth (i.e., a geometric surface).

In step 4, the ECU 16 compares a preset threshold with the first amount of accumulated PM, QMP1, estimated using the first map data, and then determines whether to initiates “the process of estimating a second amount of accumulated PM,” in which a second amount of accumulated PM, QPM2, is estimated based on the increase in the temperature at the time of temporary heating the accumulated PM.

When a determination is made that the first amount of accumulated PM, QPM1, has not reached the threshold (i.e., when a determination is made that the amount of PM is not so large that the burning process is unnecessary), the process returns to step 1.

Conversely when a determination is made that the first amount of accumulated PM, QPM1, has reached the threshold (i.e., when a determination is made that it is necessary to heat the accumulated PM), the process proceeds to step 5 to perform “the process of estimating the second amount of accumulated PM,” in which the amount of accumulated PM, QPM 2, is estimated based on an increase in temperature at the time of heating of the accumulated PM.

There will next be described “the process of estimating a second amount of accumulated PM,” in which the amount of accumulated PM is estimated based on the increase in the temperature at the time of heating of the PM.

When the post injection control is initiated, a predetermined uncombusted fuel is supplied to the DPF 12 together with exhaust gas. As the catalyst held on the DPF 12 generates reaction heat, the temperature of the catalyst increases. Consequently, the exhaust temperature TAFT downstream of the DPF 12 also increases. In this case, when the amount of accumulated PM is small, the heat capacity of the DPF 12 is also small and, accordingly, the exhaust temperature TAFT increases relatively sharply, as indicated by line L3 in FIG. 4. When the amount of accumulated PM is large, the heat capacity also increases and, accordingly, the exhaust temperature TAFT increases gently, as indicated by line L4 in FIG. 4. The reference symbols α1 and α2 each correspond to the degree of increase in temperature for each fixed unit of time, in other words, the rate at which the temperature increases.

Therefore, a substantially accurate estimate can be obtained by estimating the amount of accumulated PM based on map data (i.e., the second map data described below) created through experiment and stored in the memory. The map data represent the correlation between the amount of accumulated PM and the increase in the temperature of the exhaust temperature TAFT after a fixed time t has elapsed from the initiation of post injection control.

To be specific, in step 6, the increase in the temperature of the exhaust temperature TAFT after the fixed time t has elapsed from the initiation of the control of the post injection is calculated based on the value detected by the exhaust temperature sensor 15; and in step 7, the second amount of accumulated PM, QPM2, is estimated based on the calculated value (i.e., the increase in the temperature of the exhaust temperature TAFT) and based on the second map data as shown in FIG. 5.

In step 8, based on the pressure difference ΔP1 and the second amount of accumulated PM, QPM2, the ECU 16 corrects the gradient of the line L1 (OLD) of the first map data shown in FIG. 3, and stores it in the memory as a new line L1 (NEW). The process then returns to step 1.

Where the first amount of accumulated PM, QPM1, is different from the second amount of accumulated PM, QPM2, estimating the amount of accumulated PM by using the line L1 (OLD) of the first map data results in error in calculating a value (i.e., the first amount of accumulated PM, QPM1). In order to remove this error, the gradient of the line L1 (OLD) must be corrected according to the pressure difference ΔP1 and the second amount of accumulated PM, QPM2. Specifically, the gradient of the line L1 (OLD) is corrected so that the amount of accumulated PM when the pressure difference ΔP1 is calculated equals the second amount of accumulated PM, QPM2. The corrected line, serving as a new line L1 (NEW), is stored (updated or overwritten) in the memory.

When the gradient of the new line L1 (NEW) exceeds the threshold, the ECU 16 determines that malfunctions have arisen in the first and second exhaust pressure sensors 13 and 14 or in the exhaust temperature sensor 15 and the previous line L1 (OLD) is stored in the memory as it was.

In the foregoing exhaust air purifier for an internal combustion engine according to the present invention, the line L1 (OLD) of the first map data for “the process of estimating the first amount of accumulated PM” is corrected, upon the execution of “the process of estimating the second amount of accumulated PM,” according to the difference ΔP1 between the exhaust pressures upstream and downstream of the DPF 12, and according to the second amount of accumulated PM, QPM2, calculated through “the process of estimating the second amount of accumulated PM.” This further improves the accuracy of the first amount of accumulated PM, QPM1, estimated by “the process of estimating the first amount of accumulated PM” performed subsequently. Therefore, comparing the first amount of accumulated PM, estimated using the first map data (NEW version), with the predetermined threshold makes it possible to more accurately determine whether to estimate the second amount of accumulated PM based on a temperature increase at the time of the temporary heating of the accumulated PM. This reduces the frequency of the estimation (which consumes energy) of the second amount of accumulated PM. Accordingly, this makes it possible to provide an internal combustion engine with an exhaust air purifier that consumes less energy. 

1. An exhaust air purifier for an internal combustion engine, the purifier comprising: a particulate matter collecting unit that collects particulate matter from exhaust gas from the internal combustion engine; an exhaust pressure detecting unit that detects exhaust pressures before and after the particulate matter collecting unit; an exhaust temperature detecting unit that detects exhaust temperature downstream of the particulate matter collecting unit; a heating unit that increases the temperature of the exhaust gas passing through the particulate matter collecting unit; and a control unit that estimates the amount of accumulated particulate matter collected by the particulate collecting unit, and controls the temperature of the heating unit, wherein the control unit estimates a first amount of accumulated particulate matter based on the difference between the exhaust pressures before and after the particulate matter collecting unit, which difference has been calculated using values detected by the exhaust pressure detecting unit, and based on first map data pre-stored in a memory, which map data represent the interrelation between the amount of accumulated particulate matter in the particulate matter collecting unit and the difference between the exhaust pressures before and after the particulate matter collecting unit; when the first amount of accumulated particulate matter has reached a threshold, the control unit causes the heating unit to temporarily heat the particulate matter accumulated in the particulate matter collecting unit and also estimates a second amount of particulate matter based on an increase in temperature at the time of heating of the particulate matter, and based on second map data pre-stored in the memory, which map data represent the interrelation between the increases in the temperature at the time of the heating of the particulate matter and the amount of accumulated particulate matter in the particulate matter collecting unit; and based on the second amount of accumulated particulate matter and based on the difference between the exhaust pressures before and after the particulate matter collecting unit, the difference having been calculated based on the values detected by the exhaust pressure detecting unit, the control unit corrects the first map data representing the interrelation between the amount of accumulated particulate matter in the particulate matter collecting unit and the difference between the exhaust pressures before and after the particulate matter collecting unit. 